1. Field of the Invention
The present invention relates generally to reflow soldering and, more specifically, to a soldering method of a non-aqueous electrolyte secondary battery.
2. Background Art
A coin-type (button-type) nonaqueous-electrolyte secondary-battery has become more popular as a power supply for backup of devices because of its characteristics such as high energy density and lightweight.
When a secondary battery is used for a memory backup supply, in many cases, the battery is welded with a terminal for soldering, and then soldered on a printed circuit board together with a memory element. The soldering on the printed circuit board has been carried out using a soldering iron. However, since the number of electronic components mounted on a fixed area in the printed circuit board has been required to be increased with progress of miniaturization of devices or improvement in device performance, it has been difficult to secure a space for inserting the soldering iron. In addition, automation of soldering operation has been required for cost reduction.
Thus, a method is used, wherein soldering cream has been previously applied on a soldering area on the printed circuit board and components are placed thereon, or soldering globules are fed to the soldering area after the components have been placed, and then the printed circuit board mounted with the components is passed through a furnace in a high-temperature atmosphere, which is set such that the soldering area is at a melting point of the solder or more, for example, 200 to 260° C., thereby the solder is melted for soldering (hereinafter, referred to as reflow soldering).
The conventional coin-type (button-type) nonaqueous-electrolyte secondary-battery is not designed to withstand a high temperature at which the reflow soldering can be performed, and degradation or damage occurs in the battery due to the high temperature. Recently, a battery adaptable for the reflow soldering has been invented by using a battery material having heat resistance.
For example, a battery using an electrolyte having heat resistance (see, for:example, patent literature 1), a battery using an active material for cathode having the heat resistance (for example, patent literature 2), and a battery in which surface area of the active material for cathode is limited (see, for example, patent literature 3) are given.
[Patent Literature 1]
JP-A-2000-40525 (page 2, FIG. 1).
[Patent Literature 2]
JP-A-2002-117841 (page 3, FIG. 1).
[Patent Literature 3]
JP-A-2003-17120 (page 2, FIG. 1).
However, the conventional inventions show degradation of battery characteristics such as decrease in battery capacity or increase in internal resistance of the battery due to heat treatment during the reflow soldering process. A degree of the degradation varies not only for each reflow soldering lot, but also in a same lot, therefore the nonaqueous-electrolyte secondary-batteries after the reflow soldering process may not always exhibit the same battery characteristics. Therefore, the invention aims to provide a nonaqueous-electrolyte secondary-battery in which the degradation of the battery characteristics due to the heat treatment during the reflow soldering process is small, and a reflow soldering method of the nonaqueous-electrolyte secondary-battery in which the degradation of the battery characteristics is small.